As I understand matters BASSA are seeking legal advise re breach of contract and may pursue BA to court, and they are ballotting for industrial action. The two are not reliant on each other. As long as a strike has been organised in accordance with legislation anybody can strike over any matter that bothers them. Weak tea? If BASSA are not calling a strike specifically because they believe imposition is a breach of contract, and having read UNITE pages and watched UNITE spokesmen on news channels; this dosen't appear to be the case, then there cannot be a challenge to the legality of the action.
Actually, UNITE/AMICUS/BASSA have not given a specfic reason for a ballot. I suspect all will become clear on the 2nd November.